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K. M. Gilbert, P. Guhathakurta, J. Kalirai (UCSC), M. Cooper (UCB), S. Majewski, J. Ostheimer (UVa), D. Reitzel, R. M. Rich (UCLA)
We present results from an on-going spectroscopic survey of red giant branch stars in the outer halo of M31, which is designed to study kinematics, (sub)structure, and metallicity. The spectra were obtained using the DEIMOS instrument on the Keck II telescope. Although objects were photometrically selected to have a high probability of being red giants in M31, foreground Milky Way dwarf star contaminants still constitute a significant fraction of the observed spectra in the sparse outer halo. We present a clean sample of M31 giants in the halo and showcase the methods used in selecting the red giants. A five-parameter space was used to achieve our clean sample, including radial velocity information, the strength of the Na I absorption line doublet at 8190 Angstrom, location within a (I, V-I) color-magnitude diagram, narrow-band photometry through the DD051 filter which is centered on the surface gravity-sensitive MgH/Mg b absorption features, and the near infrared (~ 8500 Angstrom) Ca II triplet line strength. Finally, we discuss the statistical properties of the M31 giant sample, and compare photometric estimates of the metallicity with spectroscopic (Ca II line strength based) estimates.
Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. We are grateful to the DEIMOS instrument team at UCSC. This work is supported in part by NSF grant AST-0307966. K. G. acknowledges support from an NSF Graduate Research Fellowship.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.